Interferometry is a family of techniques in which waves, usually electromagnetic, are superimposed in order to extract information. Interferometry may be used in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy (and its applications to chemistry), quantum mechanics, nuclear and particle physics, plasma physics, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, and optometry.
Interferometers are widely used in science and industry for the measurement of small displacements, refractive index changes and surface irregularities. In certain fields, measurement of displacements and/or surface irregularities at the picometer scale is desirable. Accordingly, interferometers with picometer level sensitivity may be beneficial. One manner of improving interferometer sensitivity is to provide an interferometer in which multiple measuring passes are made between the reference plane and target plane of the interferometer.